1. Field of the Invention
The present invention relates to a method of directly measuring noise emitted from an optical amplifier and a system used for performing a noise figure measurement.
2. Description of the Prior Art
Heretofore, optical amplifiers and optical amplifier systems using optical amplifiers have been studied and developed in the field of optical communications. Optical amplifiers directly amplify optical signals. Two types of optical amplifiers are known, optical fiber amplifiers and semiconductor laser amplifiers. The semiconductor laser amplifiers take advantage of optical amplification phenomena in semiconductor lasers.
It is well known that there are several types of optical fiber amplifiers, for example, a fiber amplifier using the non-linearity properties of a silica fiber and a rare earth doped fiber amplifier. The rare earth doped fiber amplifier comprises an optical fiber having a core material doped with a rare earth such as erbium (Er) or neodymium (Nd). The operation of this kind of amplifier is based on the fact that the output power is emission stimulated by an input signal light from rare earth ions at a high energy level. In this case, particularly, Er-doped fiber amplifiers are now suitable for practical application in the field of optical communications because, for example, a gain band can be obtained at 1.53-1.56 .mu.m. Consequently, high-performance semiconductor lasers for employment as optical sources for EDFA pumping have been developed.
FIG. 1 shows an energy level diagram for explaining the optical amplification phenomena of the Er-doped fiber amplifier. When an optical amplification medium (i.e., an Er-doped optical fiber) receives energy from an optical source (i.e., a semiconductor laser) for pumping, ions in a core of the fiber are pumped from a ground state to a pump level. Then the ions are transited downwardly from the pump level to a metastable level without optical emission. The population of atoms in the metastable level increases and exceeds the number in the ground state, a condition called population inversion. Then optical emission can occur when the ions in the metastable level are transited to the ground state. In this case, two types of downward transitions occur: a stimulated-emission step in which the transition of ions is induced by an input light; and a spontaneous-emission (SE) step in which the transition of ions occurs spontaneously without an input light. The stimulated emission step corresponds to optical amplification. The stimulated emission enables the high-speed transition of ions and optical amplification at a band width of 1 THz (terahertz) or more. Because the now highest transmission speed of signal light is 2.4 Gbit/sec. in commercial optical communications, stimulated emission offers the hope of amplifying a higher-speed signal light. In the case of spontaneous emission, on the other hand, a lower-speed atom transition can occur, compared with that of stimulated emission. The mean life of spontaneous emission (i.e., the mean life of ions) is in the range of several milliseconds to several tens of milliseconds for the Er-doped optical fiber. The spontaneous emission light (hereinafter, referred to as SE light) is amplified in the Er-doped optical fiber and outputted as an amplified spontaneous emission (hereinafter, referred as to ASE) optical power.
The setup of the conventional optical amplifier will be explained first with reference to FIGS. 2A and 2B for a more detailed explanation.
FIG. 2A is a schematic block diagram of an optical amplifier comprising a rare earth doped optical fiber 221, while FIG. 2B is a schematic block diagram of an optical amplifier comprising a semiconductor optical amplification element 222.
In these figures, the measurement setup is constituted so as to place the optical amplification medium (hereinafter referred to as the OA medium) 221 or 222 between optical isolators 223, 224 to solve the problem of causing unstable amplification. That is, an optical amplification medium 221 or 222 amplifies the input light in both directions. This means that the amplified signal light 225 and the ASE light 226 are reflected by the optical components in front and behind the OA medium 221 or 222, so that the OA medium 221 or 222 is oscillated by these lights, resulting in unstable amplification. Therefore, the optical amplifier is placed between optical isolators 223, 224 to maintain stable amplification.
The optical amplifiers shown in FIGS. 2A and 2B also comprise an optical band-pass filter (BPF) 227 just before an output port of the optical amplifier for removing undesired ASE optical power having a wavelength different from that of the signal light. If the amplified signal light is much larger than the ASE light, the BPF 227 may be omitted.
Furthermore, the optical amplifier described above should be evaluated for its performance. In general, noise figure (NF) is one of the important parameters to evaluate the amplifier's performance. The NF parameter can be written as: ##EQU1##
wherein P.sub.ASE is the ASE optical power of an optical wave form emitted from the optical amplifier, B.sub.o is the optical bandwidth of a measurement device for measuring the P.sub.ASE level, G is the gain of the optical amplifier under measurement, h is Planck's constant, and .nu. is the optical frequency of the signal light. The second term in (1) can be neglected for high gain amplifier evaluation for simplicity.
The noise figure measurement described above enables one to judge the performance (i.e., failure, performance degradation, and so on) of the optical amplifiers and also to compare their performances. Therefore, a simple and reliable system for measuring the noise figure with a high-accuracy has been demanded. However, it is impossible to directly obtain the P.sub.ASE level to be used as a constant in the equation (1) for determining the NF because of the reasons described below.
Referring to the attached figures for understanding the problems of the conventional NF measurement method, FIG. 3 shows the inside of an OA medium 221 or 222 of the optical amplifier while FIG. 4 shows variations in gain (GF) and noise figure (NF) against an input optical power level in the typical optical amplifier. The input light passing through the optical amplifier is amplified in the OA medium. In this case, an ASE light generated from the medium can be transmitted in both directions (i.e., forward and backward directions) toward an input port and an output port of the amplifier. In the case of an optical amplifier having an optical isolator, the isolator blocks the ASE light directed in the backward direction. The ASE optical power required for determining the NF is a part of the ASE light propagated in the forward direction and it must have a wavelength corresponding to that of the input light. The optical power can be generated from the optical amplifier as the sum of the amplified input light which is amplified in the OA medium and the ASE light propagated in the forward direction. Therefore, it is necessary to separate the amplified input light and the ASE light and to perform their measurements independently.
In recent years, several NF measurement methods have been proposed, for example, an optical spectrum analyzer method (OSA method), an electric test method (S. Nishi & M. Saruwatari, "Comparison of Noise Figure Measurement Methods for Erbium doped Fiber Amplifiers", ECOC '93 Tu3, 1993), a Polarization test method (J. Aspen, J. F. Federici, B. M. Nyman, D. L. Wilson, a D. S. Shenk, "Accurate noise figure measurements of erbium-doped fiber amplifiers in saturation conditions", OFC '92 THE1, 1991), a Pulse test method.
The conventional NF measurement methods will be explained with reference to FIGS. 5-8.
FIG. 5 is a schematic block diagram of a NF measurement system for explaining an example of one conventional method (the OSA method). According to this method, a laser beam from laser diode (LD) light source 51 passes through an optical amplifier (OA) 52 under measurement to generate an output to be observed by an optical spectrum analyzer 53. Though the laser beam is of a single wavelength, the Output is generated as the sum of an amplified input light and an ASE light over a wide band of wavelengths. The optical power of the ASE light (P.sub.ASE) can be estimated from the spectra of the output. That is, the method includes the steps of:
(i) overlapping two output spectra: the output spectrum of the OA under measurement at a switch-off period in which the OA does not receive the input light; and an input spectrum of the laser beam at a switch-on period in which the OA receives the input light; and PA1 (ii) estimating the level of the P.sub.ASE that has a wavelength corresponding with that of the laser beam. PA1 generating an optical pulsed signal by modulating an intensity of an input light so as to be given at a repetition rate having a period shorter than a life time of carriers or a life time of ions in a metastable level of an optical amplifier under measurement; PA1 providing the optical pulsed signal into the optical amplifier under measurement; PA1 isolating an output power (P.sub.ASE) of the optical amplifier under measurement, in which the output power is synchronized with a space period of the optical pulsed signal; and PA1 detecting a level of the output power of the optical amplifier under measurement. PA1 the output power P.sub.leakage in synchronization with the space period of the optical pulsed signal; and PA1 an output power P.sub.OUT in synchronization with the mark period of the optical pulsed signal, and then PA1 performing a quantitative determination of a leakage characteristic Iso of the measurement system by the equation: ##EQU3## PA1 the output power P.sub.ASE generated from the optical amplifier under measurement in synchronization with the space period of the optical pulsed signal; and PA1 an output power P.sub.AMP generated from the optical amplifier under measurement in synchronization with the mark period of the optical pulsed signal. PA1 the output power P.sub.ASE generated from the optical amplifier under measurement in synchronization with the space period of the optical pulsed signal; and the output power P.sub.AMP generated from the optical amplifier under measurement in synchronization with the mark period of the optical pulsed signal. PA1 the output power including an amplified optical pulsed signal and generated from the optical amplifier under measurement in synchronization with the mark period of the optical pulsed signal may be coincident with a polarized plane of the amplified pulsed signal, and PA1 the output power including an amplified optical pulsed signal and generated from the optical amplifier under measurement in synchronization with the space period of the optical pulsed signal may be coincident with a polarized plane of the amplified pulsed signal, where these output powers are independently measured. PA1 an input terminal connecting with an input port of the optical amplifier under measurement; PA1 an output terminal connecting with an output port of the optical amplifier under measurement; PA1 a supply means for supplying an optical pulsed signal into the input terminal, the optical pulsed signal is an intensity-modulated signal given at a repetition rate having a period sufficiently shorter than a life time of carrier or a life time of ions in a metastable level of the optical amplifier under measurement; PA1 a detection means for separating and detecting: PA1 an output power P.sub.ASE generated from the output terminal in synchronization with a space period of the optical pulsed signal; and an output power P.sub.AMP generated from the output terminal in synchronization with the mark period of the optical pulsed signal on.
As the P.sub.ASE level cannot be measured directly, the results tend to vary with each observer. In this case, for example, it is necessary to use a computer or the like for fitting one spectrum with another to estimate the P.sub.ASE level of each optical amplifier by a series of measurements made automatically on a large scale.
FIG. 6 is a schematic block diagram of a NF measurement system for explaining a second preferred embodiment of the conventional methods (the Polarization test method). According to this method, a polarization controller 54 and a polarizer 55 are arranged between an optical amplifier (OA) 52 under measurement and an optical spectrum analyzer 53. In this arrangement, a laser beam from an LD light source 51 passes through the OA 52 to generate an output to be observed by an optical spectrum analyzer 53 in the same manner as that of the OSA test method (FIG. 5). In this method, furthermore, the polarization controller 54 must be regulated so as to make a single polarized light condition of the amplified laser light intersect perpendicular to the transmission characteristic of the polarizer 55. That is, the regulation should be performed so as to minimize the level of the laser light power received by the optical spectrum analyzer 53. If the polarized light condition is a single one and intersects perpendicular to the transmission characteristic of the polarizer 55, transmission of the amplified laser light can be prevented. However the ASE light is not polarized, so that its output level can be reduced to half by passing through the polarizer 55. Ideally, the optical spectrum analyzer 53 should only observe the spectrum excepting the amplified laser light to determine the P.sub.ASE level from the result of measuring the optical power of the wavelength corresponding with that of the laser beam.
The second preferred embodiment demands skill in the operator because it is hard to regulate the polarization controller 54. Therefore, it should not be applied in the NF measurements at a stage of the manufacturing process for each of the optical fibers which are mass-produced. To solve this problem, it has been suggested that a control means be employed which automatically adjusts the polarization. However, this method cannot be used to make NF measurements in practice because it increases the complexity of the apparatus and method.
On the other hand, the pulse test method utilizes the fact that the time constant of the Er-doped optical fiber is relatively long. This method includes the steps of providing a pulsed light input to the OA and measuring each level of an amplified spontaneous emission (ASE) light and a signal light by means of an OE converter having time windows. In accordance with the pulse test method, in general, the system configuration for the measurement can be simplified in spite of performing the measurement quickly with a high accuracy. Further the pulse test method has some peculiarities, for example it is not susceptible to a polarization hall burning, a single polarization, and a broad width of optical source spectrum.
Furthermore, the conventional pulse test method will be explained in detail with reference to the attached drawings.
FIG. 7 is a schematic block diagram of the setup for the conventional pulse test method and FIG. 8 is a wave form illustrating an optical surge observed in the setup of FIG. 7 (see references: D. M. Bney and J. Dupre, "Pulsed-source technique for optical amplifier noise figure measurement", ECOC '92, p509, 1992; and Klar and W. E. Heinlein, "Noise figure measurement of erbium-doped fibers amplifiers with a pulsed signal source", Technical Digest of 2nd optical fibers measurement conference, pp. 105-108, 1993 (Torino)). In this method, a rectangular on-off input signal is subjected to low-speed modulation so as to have a switching frequency of about 100 Hz, which is sufficiently lower than the time constant of the optical amplifier. At the moment of switching the signal input off, the ASE level at the output port of the OA remains at the stationary state corresponding to the level observed in the period of the switching on. Therefore, it is possible to calculate the signal light power and ASE optical power (P.sub.ASE) independently.
However the conventional pulse test method causes the following troubles. That is, if the switching-off period is not sufficiently short (i.e., switching delay), as shown in FIG. 7, the ASE light can be recovered to some extent before the input signal level disappears completely. Therefore it is acceptable to estimate the P.sub.ASE level from an over-responded wave of the ASE light. As shown in FIG. 8, furthermore, there is a sudden increase as a peak output (i.e., optical surge) in a risen-up portion of a pulse of the output light. In this case, the magnitude of the peak output depends on the input power and the linear gain, so that it may be on the order of several watts. Consequently, a part of the optical amplifier may be damaged by the peak output.